Periodicity and its breaking in compressed ferroelectric dabco monosalts

1,4-Diazabicyclo[2.2.2]octane (dabco) monosalts form a fascinating group of compounds, which exhibit exceptional dielectric properties. They can be described by a general formula: dabcoHX, where X= Br^-, I^-, ClO₄^-, BF₄^- and ReO₄^-. DabcoHI is the first material for which anisotropic relaxor properties were reported.[1] The ferroelectric relaxors are particularly desired for numerous practical applications in electronics, including miniature electronic devices and new methods of storing electricity in capacitors. Moreover, organic relaxors are environment friendly and easier to produce and to dispose off, than inorganic ceramic relaxors of mixed perovskites and doped with lead. DabcoHI undergoes a large number of transformations at elevated pressure and temperature, however analogous dabcoHBr exists only in three forms. Exceptionally rich phase diagram of ten phases was also revealed for dabcoHClO₄. At normal conditions in dabco monosalts the dabcoH⁺ cations are NH⁺···N bonded into linear chains, but pressure can considerably modify that pattern, and the weak hydrogen bonds are relevant for the formation of new polymorphs at high pressure.[2,3] In all dabcoHI polymorphs the NH⁺···N bonded chains of dabcoH⁺ cations are retained, however the chains are linear at 0.1 MPa and high pressure induces modulation of the chain with gradually increased periodicity. In dabcoHBr only phase III is NH⁺···N bonded. Although phase II of dabcoHClO₄ isostructural with phase II of dabcoHBF₄, it behaves differently on lowering temperature than it was observed in dabcoHBF₄. The polymorphic structures of dabco salts differ mainly in the arrangement of the chains and iodide anions, in the dabco conformation, and in the location of protons. When crystallized from methanol, above 1.70 GPa, dabcoHI forms solvates, and the high-pressure crystallization from aqueous solution leads to hydration already at 0.50 GPa.

Temperature variable high-pressure volumetric studies of diethylene glycol

It was demonstrated that direct compression experiments in a piston-cylinder press provide precise information on volume data and phase transition pressure complementary to X-ray diffraction. The experimental setup, described in detail previously [1], is capable of compressing liquid and solid samples up to ca. 2 GPa. We have continued these studies adding temperature control of the sample. Piston and cylinder chamber was placed in an thermally isolated mantle. The constant temperature was maintained by circulating hot air. As demonstrated by the experiments on diethylene glycol, it is a relatively quick and simple, yet efficient method for exploring phase diagrams and recording volume reduction at phase transition in different thermodynamic conditions.

Halogen- versus Hydrogen-Bonds Aggregation and Competition at High Pressure

Halogen and hydrogen bonds [1] are most often associated with the structure of molecular crystals. Even weak specific interactions, such as halogen···halogen and CH···halogen contacts, can compete between themselves and with Kitaigorodski's close packing rule. The competition between halogen···halogen and CH···halogen interactions has been studied at high pressure for the series of six dihalomethanes CH₂XY (X,Y = Cl, Br, I). They crystallize in several structural types of space groups Pbcn, C2/c, Pnma, Pna2₁ or Fmm2. In all these compounds and in their polymorphs the halogen···halogen and CH···halogen interactions persist despite considerable structural differences. The group of monohalomethanes (CH₃X, X = Cl, Br, I) are the simplest organic polar compounds and ideal models for studying halogen···halogen and CH···halogen interactions. For these simplest haloalkanes, the halogen···halogen competition with CH···halogen bonds, scaled in the function of electrostatic potential in the Cl, Br, I series, is affected by pressure. Phase α-CH₃Br, isostructural with CH₃I (orthorhombic space group Pnma) and dominated by halogen···halogen bonds, is destabilized by pressure. At 1.5 GPa the ambient-pressure α-CH₃Br phase transforms into phase β-CH₃Br governed by CH···halogen interactions. Phase β of CH₃Br is isostructural with CH₃Cl, orthorhombic space group Cmc2₁ [2,3]. The CH₃Br molecules are more evenly accommodated in space group Cmc2₁ and CH···halogen interactions are favoured by the close-packing effect.

Biomolecular systems under pressure

Biological systems are often regarded as the ultimate goal of all knowledge in this respect that they can provide the clue for understanding the origin of life and the means for improving the life conditions and healthcare. Hence the interest in high-pressure behavior of organic and biomolecular systems. Such simple organic systems were among the first structural studies at high pressure at all. They included chloroform by Roger Fourme in 1968 [1] and benzene by Piermarini et al. in 1969, still with the use of photographic technique. The efficient studies on bio-macromolecular crystals had to wait for several decades till synchrotron radiation became more accessible and Roger Fourme again stood in the avant-garde of these studies [2]. At the turn of centuries his innovations in the laboratory equipment and experimental setup let him exploring high-pressure conformations of proteins, viral capsids and the double-helix molecular architecture in nucleic acids. These directions of high-pressure studies are continued for simple and macromolecular systems of biological importance. Recently new surprising facts were revealed about the compression of urea, sucrose, and other organic compounds, as well as of macromolecular crystals. Sugars are the main energy carriers for animals as well as building blocks in the living tissue, they are also ideal models for studying pressure-induced changes of OH···O and CH···O interactions. Different types of transformations occur in compressed urea, the first organic compound synthesized in laboratory. Hen egg-white lysozyme was investigated at moderate pressure in a beryllium vessel and the compression of both tetragonal and orthorhombic modifications were measured to 1.0 GPa in a DAC; the high-pressure structure of the tetragonal form was determined and refined at still higher pressure by Fourme et al. [3] Can high pressure provide information about the remarkable polymorphism of lysozyme?